The U.S. Pat. No. 4,952,271 discloses a subassembly of components for providing an improved configuration of a friction-fusion welded joint between overlapping lengths of flexible, thermoplastic strap. FIG. 3 of that patent illustrates a lower, strap-engaging member 22 which is fixed in the base of a tool frame and which engages the lower, overlapping length of strap. A movable, upper, strap-engaging member 21 is located over the upper overlapping strap length and is adapted to press the upper strap length against the lower strap length. The two strap lengths occupy an elongate, strap-receiving path between the two strap-engaging members.
The movable strap-engaging member 21 is pivotally mounted with a pin 23 to a pair of links or arms 25 which can be actuated to move the movable, strap-engaging member 21 downwardly against the straps. A vibrator arm 41 is also pivotally mounted via the pin 23 to the strap-engaging member 21. The vibrator arm 41 is driven by an eccentric shaft for effecting oscillation of the arm 41.
The movable, strap-engaging member 21 is reciprocated by the arm 41 generally transversely of the strap in the strap-receiving path. This effects a transverse reciprocating movement of the engaged, upper, strap length against, and relative to, the lower strap length which is held stationary by the lower strap-engaging member. As a result, each strap length is heated to a sufficiently plastic condition so that a part of the thickness of each strap length at the interface region fuses. Upon cooling, a substantially continuous, re-solidified thickness of each strap length is merged with the other strap length to define a welded joint.
The subassembly disclosed in the above-discussed U.S. Pat. No. 4,952,271 provides unique advantages and benefits. Although the subassembly can be incorporated in apparatus that functions satisfactorily in various operating ranges for selected component configurations and materials, it has been found that excessive component wear can occur--especially when the components have been fabricated in particular sizes from conventional, inexpensive materials and have been operated within certain parameters. This is believed to result from substantial dynamic forces imposed upon the pivot connection that is established by the pin 23 between the vibrator arm 41 and movable, strap-engaging member 21 mounted thereon.
More specifically, the strap-engaging member 21 and the vibrator arm 41 each define one or more bores for receiving the pin 23. During the oscillation of the vibrator arm 41, the dynamic forces imposed by the arm 41 on the pin 23, and imposed by the pin 23 on the movable strap-engaging member 21, can cause excessive wear of the pin 23 and of the pin-receiving bores defined in the movable strap-engaging member 21 and the vibrator arm 41.
This wear is manifested by the receiving bores becoming non-circular in axial cross-section. That is, the axial cross sections of the holes become somewhat "oblong." Similarly, the axial cross-section of the cylindrical pin 23 becomes non-circular.
The component wear results in excessive clearance or looseness in the pivot connection. Eventually, there is a significant reduction in the reciprocating motion of the movable, strap-engaging member. The lost motion may be so great that the movement of the upper strap length is insufficient to effect a proper friction-fusion weld.
For example, in one friction-fusion apparatus of the type described above, the length of the reciprocation stroke of the movable, strap-engaging member is about 1.8 millimeters. When the components are manufactured from conventional, inexpensive materials in certain sizes and operated to effect a great number of friction-fusion welds in a particular type of thermoplastic strap, the resulting wear is eventually manifested in the creation of a clearance or lost motion of about 1.4 millimeters. This leaves only about 0.4 millimeters (1.8 millimeters minus 1.4 millimeters) of actual driven movement of the movable, strap-engaging member. Generally speaking, with a relatively small, hand-held tool employing the type of friction-fusion apparatus described above, a lost motion resulting from wear of more than about 1.0 millimeters is unacceptable from the standpoint of weld quality or consistency.
When the above-discussed friction-fusion apparatus is incorporated in a hand-held tool for use with certain types of thermoplastic strapping, it is preferred that the movable, strap-engaging member be reciprocated at relatively high frequencies. Where the strap-engaging members (e.g., members 21 and 22 in the above-identified U.S. Pat. No. 4,952,271) each define a convex surface and a concave surface merging to form a flattened "Z" or "S"-shaped configuration, relatively high oscillation frequencies on the order of 333 Hertz are preferred. The inventor of the present invention has determined that the high dynamic forces created in the apparatus at these frequencies can result in above-discussed excessive component wear--at least where the components are fabricated from relatively inexpensive, conventional materials.
In view of this problem, it would be desirable to provide an improved assembly that could accommodate the relatively high operational frequencies without subjecting the apparatus components to excessive wear. Further, it would be beneficial if such an improved assembly could accommodate the use of relatively inexpensive, conventional materials for component fabrication. Also, it would advantageous if such an improved assembly could accommodate relatively simple component manufacturing techniques and relatively simply assembly procedures.
The apparatus disclosed in the above-discussed U.S. Pat. No. 4,952,271 incorporates a special design to assist in effecting the bodily sliding movement of the upper strap length relative to the overlapped, lower strap length. In particular, the movable, upper, strap-engaging member 21 includes a plurality of downwardly projecting teeth 150. Similarly, the lower strap-engaging member 22 includes a plurality of upwardly projecting teeth 150. The teeth 150 are uniformly spaced apart in rows across the width of each strap-engaging member.
The above-described tooth design may function satisfactorily in many applications. However, unless tolerances are carefully controlled, and unless steps are taken to assure precise alignment of the components, it has been found that the teeth on the strap-engaging members can become misaligned and out of registry. In particular, it is preferred that each tooth on the movable, upper, strap-engaging member be in substantial registry with a corresponding tooth on the lower, strap-engaging member (in the initially engaged, but non-reciprocating, condition) so as to effect the desired friction-fusion weld of the overlapping strap lengths.
Owing to component wear and/or manufacturing tolerances, the initially engaged position of the movable, upper, strap-engaging member 21 relative to the lower, strap-engaging member 22 may be such that the teeth of the member 21 are offset in one direction or the other along the length of the strap-receiving path. Some of the friction-fusion welds that are produced under these initial misalignment conditions may not have the desired quality (e.g., joint strength, peel resistance, etc.). It is believed that this results from inconsistent gripping of the straps during start-up of the reciprocating motion of the movable strap-engaging member. The exact mechanism responsible for variations in weld quality as a result of the above-described tooth profile misalignment is not necessarily completely understood and there is no intent herein to be bound by any theory or explanation. Nevertheless, it would be desirable to provide an improved assembly which could accommodate slight longitudinal misalignments of the strap-engaging members and which could still produce uniformly good quality welds.
The above-discussed U.S. Pat. No. 4,952,271 also discloses a pivotally mounted cutter or knife member 51 for severing the trailing portion of the strap from the upper, overlapping strap length after the weld has been formed. The lower knife member 51 cooperates with an upper knife member 47 in severing the strap. Although, the upper and lower knife system works satisfactorily in many applications, it would be desirable to provide an improved assembly that requires fewer components and that provides increased lateral rigidity for accommodating greater forces that may encountered under certain operating conditions at high strap tension loads or with straps having larger transverse cross-sections.
The present invention provides an improved assembly which can accommodate designs having the above-discussed benefits and features.